KR20150091672A - Manufacturing method of battery separator and battery separator using the same - Google Patents

Abstract

The present invention relates to a method for manufacturing a separator and the separator manufactured thereby and, more particularly, to a method for manufacturing a separator, capable of manufacturing the separator with uniform properties by preventing the surface of the separator from being damaged in a roll extension process by forming a protection film on both sides of a precursor film formed with a single layer or a multilayer and extending the protection film.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a separator,

More particularly, the present invention relates to a method of producing a separator and a separator prepared thereby, and more particularly, to a method of forming a protective film on both sides of a single-layer or multi-layered precursor film, The present invention relates to a method for producing a separation membrane capable of producing a separation membrane having a physical property and a separation membrane produced thereby.

The polyolefin microporous film can be used as a separator for various batteries such as primary and secondary lithium batteries, lithium-polymer batteries, nickel-hydrogen batteries, nickel-cadmium batteries and nickel-zinc batteries, separation filters, and is widely used as a membrane. When used as a separator for various batteries, the performance of the separator is an important factor in the characteristics, productivity and stability of the battery. Therefore, the separator should have suitable mechanical properties, heat resistance, transparency, dimensional stability, and shut down characteristics.

A method for producing a microporous film from a polyolefin includes a method of making a microporous film in the form of a nonwoven fabric by making a polyolefin into a thin fiber, a dry method of forming a microporous film by stretching after making a thick polyolefin film, There is a wet method in which a single phase is formed, and after the phase separation, the diluent portion is extracted to make voids in the polyolefin. The double wet method is suitable as a secondary battery separator because the thickness of the film is thin and uniform as compared with the other two methods, a thin film can be made, and excellent physical properties can be obtained.

A general method of producing a microporous membrane by a wet process is disclosed in U.S. Patent No. 4,247,498 which uses a polyethylene and corresponding diluent and blends the mixture at a high temperature to make a thermodynamically uniform solution, And a polyolefin porous membrane is produced by phase-separating the polyethylene diluent in a cooling process.

The lithium ion secondary battery is an excellent battery having an extremely high energy density. However, there is a risk of explosion when a short circuit occurs, so that a separator used is required to have a high and uniform quality level. In recent years, quality stability and uniformity have been demanded more in accordance with the high capacity and high output of lithium ion secondary batteries such as batteries for hybrid vehicles. If the quality stability and uniformity of the separator are inferior, the fragile portion of the separator due to overheating of the battery is broken, and the risk of explosion due to the breakage is increased.

The polyolefin microporous membrane used as such a battery separator is mainly composed of a polypropylene single layer, a polyethylene single layer, or a three-layer separator of polypropylene / polyethylene / polypropylene.

In order to form a uniform permeability in the production of a separator, a stretching process is indispensable. In the case of stretching by a speed difference between rolls and rolls using a roll in the stretching process, friction and slip occur between the rolls and the precursor film, Scratches are generated on the surface of the substrate, or appearance problems such as wrinkles occur.

In this case, due to the surface damage of the final separation membrane, there is a large deviation in permeability, and problems such as yield and productivity deteriorate due to defective products.

Korean Patent Registration No. 0943697 (Patent Document 1) discloses a method for producing a polyethylene microporous membrane having excellent physical properties and productivity by regulating biaxial stretching conditions by rolls and then biaxially stretching.

The separation membrane having excellent physical properties can be produced by changing the longitudinal stretching conditions. However, as the number of rolls of the roll drawing equipment used increases, the surface damage of the final separation membrane product is intensified and the productivity is decreased.

Korean Patent No. 0943697

Disclosure of the Invention The present invention has been conceived to solve the problems as described above. It is an object of the present invention to provide a protective film on both sides of a precursor film, And to provide a method for producing a separation membrane having a uniform permeability deviation and improved productivity.

It is another object of the present invention to provide a separation membrane produced by the above-described production method.

According to an aspect of the present invention, there is provided a method of manufacturing a composite precursor film, comprising: a) preparing a composite precursor film to form a protective film on both sides of a precursor film; b) stretching the composite precursor film to produce a composite film; And c) removing the protective film of the composite film to obtain a separation membrane.

According to one embodiment of the present invention, the protective film includes a crystalline polymer, and may be formed as a single layer or a multilayer.

According to an embodiment of the present invention, the protective film may include a polyolefin resin.

According to one embodiment of the present invention, the precursor film may be a single layer or a multilayer film comprising a polyolefin-based resin.

According to one embodiment of the present invention, the polyolefin-based resin may be selected from polyethylene, polypropylene, and copolymers thereof.

According to an embodiment of the present invention, the protective film may be heat-treated at the crystalline polymer melting temperature (Tm ^P ) of -50 ° C to Tm ^P ° C.

According to one embodiment of the present invention, the protective film may be a polypropylene single-layer film or a multilayer film in which polypropylene and polyethylene are alternately laminated.

According to an embodiment of the present invention, the thickness of the protective film may be 10 to 40 탆.

According to an embodiment of the present invention, the stretching in the step b) may be uniaxial or biaxial stretching performed in a temperature range satisfying the following formula 1 or 2:

MIN (Tm ^S ) -200 ° C? Low? MIN (Tm ^S ) -30 ° C [Equation 1]

^{MIN (Tm S) -1 ℃ ≤} High <MIN (Tm S) -30 ℃ [ Formula 2]

In the above formula 1 and formula 2, MIN (Tm ^S ) is the lowest melting temperature (占 폚) among the polyolefin resins constituting the composite film, Low is the stretching temperature (占 폚) Drawing temperature (℃).)

According to another aspect of the present invention, there is provided a separation membrane produced by the separation membrane production method. According to an embodiment of the present invention, the coefficient of variation of the gas permeability of the separation membrane is 4% or less.

According to the method for producing a separation membrane of the present invention and the separation membrane produced therefrom, defects such as scratches and wrinkles on the surface of the separation membrane can be prevented by laminating and stretching a protective film during the production of the separation membrane and the deviation of the permeability of the final separation membrane is remarkably reduced have. In addition, there is an advantage that productivity can be improved by preventing breakage of the separator during stretching.

Hereinafter, the method for producing a separation membrane of the present invention and the separation membrane produced therefrom will be described in detail. The following embodiments and drawings are provided by way of example so that those skilled in the art can fully understand the spirit of the present invention. Further, it is to be understood that, unless otherwise defined, technical terms and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Descriptions of known functions and configurations that may be unnecessarily blurred are omitted.

The present inventors have found that, in providing a method for producing a separation membrane capable of preventing surface damage such as scratches and wrinkles, the protective film is formed on both surfaces of a precursor film and then stretched to produce a separation membrane. , The permeability deviation is reduced, and thus a separation membrane having uniform physical properties can be produced, thereby completing the present invention.

A method of manufacturing a separation membrane according to an embodiment of the present invention includes:

a) preparing a composite precursor film to form a protective film on both sides of the precursor film;

b) stretching the composite precursor film to produce a composite film; And

and c) removing the protective film of the composite film to obtain a separation membrane.

Hereinafter, each component will be described in more detail.

The step a) of the present invention is a step of producing a composite precursor film which forms a protective film on both sides of a precursor film.

The precursor film according to the present invention is not limited as long as it is a polymer resin capable of producing a separator commonly used in the related art. For example, the precursor film may include a polyolefin resin.

The polyolefin resin may be selected from polyethylene, polypropylene, and copolymers thereof, but is not limited thereto.

The polyolefin-based resin is not limited, but preferably has a weight average molecular weight ranging from 100,000 to 500,000 g / mol.

If the weight average molecular weight of the polyolefin-based resin is less than 100,000 g / mol, the permeability may be lowered. If the weight-average molecular weight is more than 500,000 g / mol, the extrusion moldability of the precursor film may be deteriorated.

An inorganic filler may be further added to the resin composition. The inorganic filler acts as the nucleus of the pore serving as the initiator of the pore at the time of stretching, and can help improve the heat resistance. The inorganic filler can be selectively used to increase the porosity and pore size of the separator to increase the permeability, and to increase the frictional force during stretching to prevent slippage. For example, calcium carbonate, silica, barium sulfate, talc, or a mixture thereof may be used, and calcium carbonate or silica is preferable considering the uniformity of the quality of the separation membrane.

If necessary, conventional additives for improving specific functions such as an oxidation stabilizer, a UV stabilizer, and an antistatic agent may further be added to the resin composition.

The resin composition containing the polyolefin resin can be kneaded using a biaxial compounder, a kneader or a Banbury mixer, and melt-extruded to produce a sheet-like precursor film. The optional components such as the polyolefin resin and the inorganic filler can be blended in advance and put into the compounder or separately from the separate feeder. The extrusion temperature may be 160 to 250 占 폚, preferably 180 to 230 占 폚, and more preferably 190 to 220 占 폚. If the extrusion temperature is less than 160 캜, the polyolefin resin may not be sufficiently melted during extrusion, which may result in a large processing load, and dispersion may not be uniform. If the extrusion temperature is higher than 250 ° C, the polyolefin-based resin may deteriorate and the molecular weight may be lowered and discolored.

The precursor film may be a monolayer precursor film or a multilayer precursor film. In addition, a single-layer precursor film may be laminated or co-extruded to form a multi-layer precursor film.

The single-layer or multi-layer precursor film of the present invention may be further produced and then further subjected to a heat treatment step. The temperature and time conditions of the heat treatment step are not limited, but it is preferably performed at a temperature of MIN (Tm ^S ) -100 ° C to MIN (Tm ^S ) -3 ° C for 1 minute to 72 hours. The MIN (Tm ^S ) is a melting temperature (캜) of a substance having the lowest melting temperature among the polyolefin resins forming the precursor film.

If the heat treatment temperature according to the present invention is lower than the above temperature range, there is a fear that the heat treatment effect is insufficient and the permeability is lowered. If the temperature range is exceeded, the orientation of crystal and crystal may be damaged and pore formation may be difficult.

The protective film according to one embodiment of the present invention is laminated on both sides of the precursor film and protects the surface damage of the separation membrane during the stretching through the roll stretching device.

The protective film is not limited as long as it is a polymer resin capable of forming a protective film ordinarily used in the related art. For example, the protective film may be a crystalline polymer.

The crystalline polymer may be a polyolefin-based resin having a melting temperature (Tm) of 120 to 170 ° C. More specifically, the crystalline polymer may be selected from polyethylene, polypropylene, and copolymers thereof, It is not.

The protective film may be composed of the same polyolefin resin as the precursor film.

When the protective film is formed by the polyolefin-based resin described above, it is possible to prevent surface damage during the production of the separator, and it is excellent in workability because of its excellent slip and easy adhesion and desorption with the separator.

The protective film according to an embodiment of the present invention is preferably subjected to heat treatment at a crystalline polymer melting temperature (Tm ^P ) of -50 ° C to Tm ^P ° C. By the heat treatment in the above-mentioned range, shrinkage and wrinkling of the protective film Or fracture can be prevented in advance.

The protective film according to an embodiment of the present invention may be formed as a single layer or a multilayer, and more specifically, a polypropylene single layer or a multilayer film in which polypropylene and polyethylene are alternately laminated, but is not limited thereto.

When the protective film is formed of the single layer or the multi-layer film, the surface damage caused during the production of the separator can be prevented, the slip property is excellent, the workability is excellent, and the manufacturing cost is low.

If the thickness of the protective film is less than 10 mu m, damage such as surface scratches generated in the protective film may be transferred to the separator and the protective film may not be sufficiently protected. When the thickness exceeds 40 mu m, There may be a problem that the occurrence of wrinkles becomes worse at the time of stretching.

As a method of forming the protective film on both sides of the precursor film, it is possible to position two rolls of the protective film at the outermost position, place one roll or more of the precursor film therebetween, .

Alternatively, the multiple layers of the film having the outermost protective film may be wound in separate rolls at the same time, and the film may be fed from the roll to the stretching machine, but the present invention is not limited thereto.

The step (b) of the present invention is a step of stretching the composite precursor film to produce a composite film.

The stretching step according to an embodiment of the present invention is not limited, but may be uniaxial or biaxial stretching including both a roll method and a tenter method.

More specifically, as described above, it is possible to stretch a plurality of films supplied to an stretching machine in a temperature range satisfying the following formula (1) or (2).

MIN (Tm ^S ) -200 ° C? Low? MIN (Tm ^S ) -30 ° C [Equation 1]

^{MIN (Tm S) -1 ℃ ≤} High <MIN (Tm S) -30 ℃ [ Formula 2]

In the above formula 1 and formula 2, MIN (Tm ^S ) is the lowest melting temperature (占 폚) among the polyolefin resins constituting the composite film, Low is the stretching temperature (占 폚) Drawing temperature (℃).)

If the temperature is outside the range of the above formula 1 at the time of low-temperature stretching, the mobility of the amorphous portion is too low or too high to form the initial pores due to tearing between crystals and crystals, There is a concern.

If the temperature is lower than the temperature range of the formula (2), the shrinkage of the separator may excessively increase. If the temperature exceeds the temperature range of the formula (2), the crystal structure may be damaged and the gas permeability may decrease.

The step c) of the present invention is a step of removing the protective film of the composite film to obtain a separation membrane.

The protective film can be separated and removed from the stretched film produced all at once from the stretching machine, and the final separation membrane can be obtained as a product. The method of desorbing the protective film can be applied without limitation without limiting the physical properties of the separator.

For example, when the stretched film is a multi-layer separating film, the separating film from which the protective film has been separately removed is wound into a separate roll, and the roll is again wound up to be a layered product using a separate monolayer to obtain a final separator as a product can do. In addition, the protective film on which the protective film has not been removed can be separated and removed on both sides of the protective film by using a separate monolayer, and the remaining separator can be obtained as a product.

The separation membrane of the present invention can be produced by the above-described production method, and the coefficient of variation (CV) of the gas permeability (Gurley permeability constant) can be 4% or less. The coefficient of variation (CV) represents a ratio to the average value of the standard deviation. The coefficient of variation (CV) can be calculated by the average value and the standard deviation of the gas permeability measured for the membrane of the present invention, and the unit is a percentage (%).

Hereinafter, the present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the following examples.

How to measure property

1) Measurement of gas permeability and gas permeability deviation

Gas permeability was measured according to JIS P8117 using a Gurley type densometer (G-B2C) manufactured by Toyoseiki. The average and standard deviation of the gas permeability were obtained by taking a membrane sample with a longitudinal length of 50 cm from the film roll and measuring the gas permeability (gurley) in 20 places by dividing into 4 parts in the transverse direction and 5 parts in the longitudinal direction. The standard deviation was calculated and the coefficient of variation was calculated from the following equation.

[Mathematical Expression]

Coefficient of Variation (CV) = standard deviation / mean value X 100 (%)

 [Example 1]

Using a polypropylene resin (weight average molecular weight 400,000 g / mol) having a melt index (2.16 kg, 230 ° C) of 2 g / 10 min, a precursor film having a thickness of 22 μm was extruded by casting at a winding speed of 30 m / And heat treated at 135 ° C for 12 hours.

The polypropylene single-layer protective film having a thickness of 8 占 퐉 was heat-treated at 135 占 폚 for 12 hours.

The above film was laminated in three layers by the structure of protective film / precursor film / protective film and stretched. The film was stretched 10% longitudinally at a low temperature stretching temperature of 25 占 폚 and then stretched 200% longitudinally at 130 占 폚 at a high temperature stretching temperature.

The protective film was removed from the thus-produced separator, and the remaining separator was produced as a product. The Gurley gas permeability average was 164 sec / 100cc, the Gurley gas permeability standard deviation was 7.0 sec / 100cc, and the Gurley gas permeability coefficient was 4.3%.

[Example 2]

The procedure of Example 1 was repeated except that the protective film had a thickness of 11 탆. The Gurley gas permeability was 152 sec / 100cc, the Gurley gas permeability standard deviation was 3.2 sec / 100cc , and the Gurley gas permeability coefficient was 2.1%.

[Example 3]

The procedure of Example 1 was repeated except that the thickness of the protective film was 20 占 퐉. The Gurley gas permeability average of the prepared membranes was 148 sec / 100cc, the Gurley gas permeability standard deviation was 3.4 sec / 100cc, and the Gurley gas permeability coefficient was 2.3%.

[Example 4]

A protective film was used in the same manner as in Example 2 except that the protective film was not subjected to heat treatment. In this case, the protective film was not uniformly stretched during stretching, and wrinkles were formed. The Gurley gas permeability average was 160 sec / 100cc, the Gurley gas permeability standard deviation was 4.0 sec / 100cc, and the Gurley gas permeability coefficient was 2.5%.

[Comparative Example 1]

Using a polypropylene resin (weight average molecular weight 400,000 g / mol) having a melt index (2.16 kg, 230 ° C) of 2 g / 10 min, a precursor film having a thickness of 22 μm was extruded by casting at a winding speed of 30 m / And heat treated at 135 ° C for 12 hours.

The precursor film was stretched by 10% in the longitudinal direction at a low temperature stretching temperature of 25 占 폚 and then stretched by 200% in the longitudinal direction at a high temperature stretching temperature of 130 占 폚 without laminating the protective film.

The Gurley gas permeability average was 174 sec / 100cc, the Gurley gas permeability standard deviation was 18.2 sec / 100cc, and the Gurley gas permeability coefficient was 10.5%.

[Example 5]

(Weight average molecular weight: 400,000 g / mol) having a melt index (2.16 kg, 230 DEG C) of 2 g / 10 min and a polyethylene resin having a melt index (2.16 kg, 190 DEG C) of 0.3 g / layered precursor film of polypropylene resin / polyethylene resin / polypropylene resin having a thickness of 25 占 퐉 was co-extruded and cast at a winding speed of 30 m / min. The interlayer thickness ratio of the cast precursor film was 1: 1: 1. The film was heat-treated at 125 DEG C for 12 hours.

The polypropylene single-layer protective film having a thickness of 20 占 퐉 was heat-treated at 135 占 폚 for 12 hours.

The above film was laminated in three layers by the structure of protective film / precursor film / protective film and stretched. 25% stretching in the longitudinal direction at a low temperature stretching temperature of 25 占 폚 and 160% stretching in the longitudinal direction at a high temperature stretching temperature of 120 占 폚.

The protective film was removed from the thus-produced separator, and the remaining separator was produced as a product. The Gurley gas permeability average was 234 sec / 100cc, the Gurley gas permeability standard deviation was 5.8 sec / 100cc, and the Gurley gas permeability coefficient was 2.5%.

[Example 6]

The protective film was formed in the same manner as in Example 5 except that a three-layer film of polypropylene / polyethylene / polypropylene having a thickness of 20 탆 and a layer ratio of 1: 1: 1 was used after heat treatment at 125 캜 for 12 hours. The Gurley gas permeability average of this membrane was 221 sec / 100cc, the Gurley gas permeability standard deviation was 6.4 sec / 100cc, and the Gurley gas permeability coefficient was 2.9%.

[Comparative Example 2]

(Weight average molecular weight: 400,000 g / mol) having a melt index (2.16 kg, 230 DEG C) of 2 g / 10 min and a polyethylene resin having a melt index (2.16 kg, 190 DEG C) of 0.3 g / layered precursor film of polypropylene resin / polyethylene resin / polypropylene resin having a thickness of 25 占 퐉 was co-extruded and cast at a winding speed of 30 m / min. The interlayer thickness ratio of the cast precursor film was 1: 1: 1. The film was heat-treated at 125 DEG C for 12 hours.

The heat-treated precursor film was stretched by 25% in the longitudinal direction at a low-temperature stretching temperature of 25 占 폚 and stretched by 160% in the longitudinal direction at a high-temperature stretching temperature of 120 占 폚 without laminating the protective film.

The Gurley gas permeability average was 342 sec / 100cc, the Gurley gas permeability standard deviation was 43.8 sec / 100cc, and the Gurley gas permeability coefficient was 12.8%.

[Table 1]

As shown in Table 1, in Examples 1 to 6, when the protective film was used, the transmittance was improved in Comparative Examples 1 and 2 as compared with the case where the protective film was not used, and the standard deviation of the transmittance and the coefficient of variation Respectively.

In addition, as in Example 1, although the protective film is used, the effect of the protective film is small, and the standard deviation and coefficient of variation of the Gurley gas permeability are drastically increased when the thickness is less than 10 mu m . As in Example 4, there was no significant difference in the standard deviation and coefficient of variation of Gurley gas permeability versus Gurley gas permeability in the case of using a heat-treated protective film in the case of using a heat-treated protective film, There is a problem that wrinkles are generated because they are not stretched.

[Table 2]

As shown in Table 2, when the protective film was applied to the case where the precursor film was a three-layer film (Example 5) and when the protective film was a three-layer film (Example 6) The Gurley gas permeability standard deviation and the coefficient of variation were significantly decreased compared with the Gurley gas permeability.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, Various modifications and variations are possible in light of the above teachings.

Accordingly, the spirit of the present invention should not be construed as being limited to the embodiments described, and all of the equivalents or equivalents of the claims, as well as the following claims, belong to the scope of the present invention .

Claims (11)

a) preparing a composite precursor film to form a protective film on both sides of the precursor film;
b) stretching the composite precursor film to produce a composite film; And
c) removing the protective film of the composite film to obtain a separation membrane.
The method according to claim 1,
Wherein the protective film comprises a crystalline polymer and is formed as a single layer or a multilayer.
3. The method of claim 2,
Wherein the protective film comprises a polyolefin-based resin.
The method according to claim 1,
Wherein the precursor film is a single layer or a multilayer film comprising a polyolefin resin.
The method according to claim 3 or 4,
Wherein the polyolefin-based resin is at least one selected from the group consisting of polyethylene, polypropylene, and copolymers thereof.
3. The method of claim 2,
Wherein the protective film is heat-treated at a temperature of the crystalline polymer melting point (Tm ^P ) of -50 ° C to Tm ^P ° C.
The method of claim 3,
Wherein the protective film is a polypropylene single-layer film or a multilayer film in which polypropylene and polyethylene are alternately laminated.
The method according to claim 1,
Wherein the thickness of the protective film is 10 to 40 占 퐉.
The method according to claim 1,
Wherein the stretching in the step (b) is performed in a temperature range satisfying the following formula (1) or (2).
MIN (Tm ^S ) -200 ° C? Low? MIN (Tm ^S ) -30 ° C [Equation 1]
^{MIN (Tm S) -1 ℃ ≤} High <MIN (Tm S) -30 ℃ [ Formula 2]
In the above formula 1 and formula 2, MIN (Tm ^S ) is the lowest melting temperature (占 폚) among the polyolefin resins constituting the composite film, Low is the stretching temperature (占 폚) Drawing temperature (℃).)
A separation membrane produced by the separation membrane production method according to any one of claims 1 to 4 and 6 to 9.
11. The method of claim 10,
Wherein the separation membrane has a coefficient of variation (CV) of the gas permeability (Gurley permeability constant) of 4% or less.